Metal matrix composites ("MMC's) are structurally reinforced materials which normally exhibit superior properties when compared to corresponding unreinforced materials. MMC's are attractive materials for engine components, electronic packaging, and many other structures.
One common method for fabrication MMC's is liquid metal infiltration. Liquid metal infiltration is a process in which liquid metal is permeated into a porous preform of reinforcement material. Upon subsequent solidification of the impregnated preform, the metal becomes the matrix of the resulting composite. Liquid metal infiltration processes include squeeze casting, pressure infiltration, and variations thereof.
The quality of the composites made by liquid metal infiltration is governed not only by the material used for the reinforcement, but also by how these reinforcement materials are binded together as a preform. For example, to obtain near-net-shaped MMC's (i.e., MMC's shaped essentially the same as before metal infiltration), the preform must have sufficient strength to withstand the applied pressure with which liquid metal is forced into the preform interstices. Preform strength is dictated in large degree by the particular binder utilized and the method of preform fabrication. In spite of the importance of binders and preforms in liquid metal infiltration, relatively little research has been conducted on alternative binders for preform processing.
Polymer binders, such as polyvinyl alcohol (PVA), are widely used at room temperature, but cannot withstand temperatures above about 200.degree. C. A variety of binders have been used in ceramic fabrication processes, including oxysulfates, oxychlorides, sodium silicate and hydraulic cements. In contrast to polymer binders, these ceramic binders have high strength at room temperature, but typically result in unsatisfactory low strength when heated to temperatures of 500.degree.-800.degree. C. Consequently, these binders are not suitable for making preforms for many MMC's, because, during metal infiltration, the preform must commonly be heated to temperatures above 200.degree. C. There is thus a need for binders which result in MMC's having superior strength and other properties.
Silica colloid binders are widely used binders for preforms used to fabricate MMC's. However, difficulties are encountered using this binder with some reinforcement materials and metal matrices. For example, carbon and SiC reinforcement materials have a tendency to react with aluminum metal to form aluminum carbide, Al.sub.4 C.sub.3, and silicon, both of which decrease the performance of the resultant MMC. Thus, the need remains in the art for binders which alleviate the difficulties encountered using SiC and/or carbon fiber reinforcements.